The injection nozzle front end section of an injection molding machine is in contact with a metallic mold, which is cooled to a low temperature. The injection nozzle is consequently apt to be cooled, too. In order to prevent this, the injection nozzle itself is conventionally heated separately, independently of the heating cylinder. Since, however, this separate heating is performed with a single heating device such as a coil heater, the following problems are involved:
Since the injection nozzle end is in contact with a metallic mold, which is set at a temperature much lower than the melting point of molten material such as plastics, its temperature is lower than the set value of the injection nozzle temperature normally by several tens of degrees C. In the rear section of the injection nozzle, on the other hand, there is heat transfer from the heating cylinder. This situation, along with the low heat radiation from the injection nozzle, makes the nozzle rather overheated, resulting in an uneven heat distribution over the nozzle itself. This is readily appreciated from FIG. 3, which shows the result of an investigation into the heat distribution of an injection nozzle. In FIG. 3, the two-dot chain line A represents the heat distribution when the temperature control is effected by detecting the temperature in the rear section of the injection nozzle. The dashed line B represents the heat distribution when the temperature is controlled with a temperature detecting apparatus provided in the vicinity of the injection nozzle front end.
The temperature control is thus performed with only one system for detecting and controlling nozzle temperature, inspite of the temperature difference of several tens of degrees C. between the front end and the rear section of an injection nozzle. Setting the nozzle front end to an appropriate temperature consequently results in a raised temperature in the nozzle rear section, which causes the resin to be overheated until it is "burned". In addition, the resin pushed out through the nozzle drips from the nozzle opening, presenting a phenomenon called "stringing" or "drooling". When, on the other hand, the temperature setting is conducted to the nozzle rear section, the tip resistance is increased, augmenting the pressure loss, a situation which leads to increased variation in molding.
In view of the above problems, the present invention aims at providing an injection nozzle for an injection molding machine which prevents the drooling or stringing of resin from the injection nozzle front end as well as resin burning, and which is capable of stabilizing products and reducing pressure loss by keeping the injection nozzle front end section having a narrowed passage at an appropriate temperature.